Strap spring for attaching heat sinks to circuit boards

ABSTRACT

A strap spring for insertion into a circuit board for securing a heat sink to an electronic device is disclosed that has a body portion having two ends and a contact section and one or more engaging sections extending from said ends. The ends are inserted into an aperture in the circuit board and an aperture in the heat sink to anchor the circuit board to the heat sink. Electronic assemblies comprising a circuit board having a microprocessor and a heat sink having a surface larger than the microprocessor are also disclosed in which a strap spring is disposed on a side of the circuit board opposite the heat sink, and the strap extends through the circuit board to engage the heat sink.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.08/677,596, filed Jul. 9, 1996, now U.S. Pat. No. 5,847,928.

FIELD OF THE INVENTION

The present invention relates to securing electronic componentstogether. More particularly, it relates to a strap spring for attachinga heat sink to an electronic device on a printed circuit board.

BACKGROUND OF THE INVENTION

A heat sink placed in contact with an electronic device transfers heatthrough conduction from the semi-conductor device contained in theelectronic package. U.S. Pat. No. 4,884,331—Hinshaw shows a commonlyused pin fin heat sink used to dissipate the conducted heat.

Springs or clips, such as those shown in U.S. Pat. No.4,745,456—Clemens, or U.S. Pat. No. 4,660,123—Hermann, have been used tosecure heat sinks to electronic devices, and a catalog entitled“Thermalloy Semiconductor Accessories,” pp. 15-29, Thermalloy, Inc.,Dallas, Tex. (USA) also shows clips for attaching heat sinks toelectronic devices. Page 17 of the Thermalloy catalog shows two types ofwire springs for securing a heat sink to a pin grid array (PGA).Co-pending application Ser. No. 349,672, filed Dec. 5, 1994, “Strap ForHeat Sink Clip Assembly,” shows and describes several different clipsfor securing a heat sink to an electronic device. Another type of anchorfor securing a heat sink to a printed circuit board is shown inco-pending application Ser. No. 08/477,794 filed Jun. 7, 1995. Both ofthese co-pending applications are incorporated herein by reference as ifset forth in their entirety.

However, if a heat sink is larger than the microprocessor there is nosecure way to attach the heat sink to the microprocessor. Themicroprocessor is typically, but not necessarily, a socket. Surfacemount applications for Ball Grid Array (BGA), Column Grid Array (CGA),Land Grid Array (LGA), Plastic Quad Flat Pack (PQFP), are all examplesof devices that are not in sockets.

In the above-mentioned prior art attachment means, which clip the heatsink to the circuit board, the circuit boards often warp due to thespring loading by the attachment device. Therefore, devices that provideattachment to the board normally have to be kept close to themicroprocessor, so that the board is not put under large bendingstresses. Also, some prior art clips require a soldering operation toaffix the hook in the board. This usually would be a wave solderingoperation, which is not normally done when surface mount technology isused.

It is therefore an object of the present invention to provide an anchorwhich can be inserted through a printed circuit board to secure a heatsink and an electronic device to the board.

SUMMARY OF THE INVENTION

The foregoing and other objects, features and advantages of theinvention will be better understood from the following more detaileddescription and appended claims. It has now been found that thedeficiencies of the prior art can be overcome by a strap spring in whichthe spring contacts the circuit board directly below the microprocessor.All of the spring loading is in compression through the circuit board isconcentrated at the microprocessor. The ends of the clip snap throughholes or slots in the heat sink to attach the entire assembly together.Therefore, the board is allowed to “float” and no bending moment isapplied. The strap spring of the present invention can be constructed inembodiments that are easily removable by dislocating the inside comersof the attaching portions and popping them back through the holes.Embodiments of the present invention are also disclosed that have barbsor other means to snap through the board on two sides of the clipthereby locking the clip into the heat sink. Since the strap spring ofthe present invention easily snaps the assembly together, it can beeasily automated for high production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view of the strap spring of the presentinvention;

FIG. 2 is a top view, broken away, of the strap spring illustrated inFIG. 1;

FIG. 3 is an exploded perspective view of the strap spring of thepresent invention and related components;

FIG. 4 is a perspective view of an assembled heat sink and electronicdevice anchored with a strap spring made in accordance with the presentinvention;

FIG. 5 is a cross-section of the assembly shown in FIG. 4, taken alongline 4—4 of an assembly securing a heat sink to a printed circuit board;

FIG. 6 is a top plan view, similar to FIG. 2, of the assembly of FIG. 5;and

FIG. 7 is an elevation view in cross-section, similar to FIG. 5, showingan alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a side elevation view of a strap spring 100made in accordance with the present invention is illustrated. The strapspring 100 is generally comprised of two arms 110, 112 that extend froma central section 120. The arms 110,112 each have an end and arepreferably terminated with engagement sections 130,132. As explained infurther detail below, various embodiments of the engagement section canbe designed to permit the strap spring 100 to either be easily remove orto bZe permanently or semi-permanently affixed in place. In thepreferred embodiment shown in FIG. 1, the central section 120 isconnected to the arms 110,112 at a slight bend to create the flattened“V” shape illustrated. This design provides a secure mounting due to thelocalized force created, as explained below.

FIG. 2 is a top plan view showing the detail of one of the ends 130 ofthe embodiment of the strap spring 100 that is illustrated in FIG. 1.Referring to both FIG. 1 and FIG. 2, it can be seen that in thisembodiment, the flat strap that forms the arm 110 is bent into anarcuate shape. As will be recognized by those of skill in the art, sucha shape adds rigidity and strength to the engagement portion. Althoughan arcuate shape is generally preferred, it will be further realizedthat any number of shapes, e.g., square, “V” shapes, curled edges, etc.can be used in place of the arcuate “U” shape shown in FIG. 2. As alsoseen in FIGS. 1 and 2, the engagement portion has a generallywedge-shaped profile with an undercut surface 140 that engages the heatsink, as explained in further detail below.

The strap spring described above is used in an assembly along with othercomponents. Thus, referring now to FIG. 3, an exploded view of a typicalassembly is illustrated. In FIG. 3, a heat sink 10, circuit board 20 andstrap spring 100 are shown. The circuit board 20 is cut away to betterillustrate the assembly. A socket 22 is attached to the circuit board20, and a device 50, in the illustrated example a BCA chip, is insertedinto the socket 22. Referring now to FIG. 4, an illustration of thepreferred embodiment shown in FIG. 3 is provided. As shown, the strapspring 100 is preferably disposed on the side of the circuit board 20that is opposite the heat sink 10. The engagement portions 120,122extend through apertures in the circuit board 20, and through apertures11 in the heat sink 10. Engagement of the undercut surface 140, combinedwith the force created by the flexure of the arms (not visible) combineto lock the assembly together. However, because the undercut surface 140presents basically a sliding surface to the surface of the heat sink 10,the engagement is not permanent, and the heat sink 10 can be removedwith relatively little effort by displacing the engagement sections130,132.

Further details of the assembly illustrated in FIG. 4 are seen in thecross-sectional view of FIG. 5, which is taken along line 5—5 in FIG. 4.As seen therein, in preferred embodiments of the present invention, themicroprocessor 50 is smaller than the heat sink 10, although it ispossible for it to be larger in other embodiments. However, one problemsolved by the present invention is its ability to provide an attachmentdesign that can effectively retain an assembly that retains amicroprocessor that is smaller than the heat sink without creating unduebending stresses and creating a stable thermal interface by mechanicallyforcing the microprocessor 50 into the heat sink 10. As seen in FIG. 5,the engagement portions 130,132 are “outboard” of the microprocessor 50,while the upwardly bent central section 120 ensures that any forcesgenerated are concentrated near the microprocessor 50.

As mentioned above, the undercut surfaces 140 engage the circuit board20 and resist the forces described immediately above. This engagement isfurther illustrated in the top plan view shown in FIG. 6. As notedabove, the engagement may be one of degree, and can be easilydisengaged, semi-permanent, or even permanent if desired.

An alternate embodiment of the strap spring 100 of the present inventionis shown in FIG. 7. In those instances where it is desired to more orless permanently secure the heat sink 10 to the microprocessor, a barb140′ can be provided at the end 130 of the strap spring 100. In theillustrated embodiment, two barb points 141 are provided. It will beunderstood that the number, size and geometry of the barbs may be variedto provide proper securement based upon the design of the heat sink andthe degree of security required.

The present invention thus also discloses improved methods of assemblingelectronic components in an electronic device is placed in a socketmounted to a circuit board and a heat sink is placed on the electronicdevice. In accordance with the invention, a strap spring is pushedthough apertures in the circuit board, and then through apertures in theheat sink that are in registration with holes in the heat sink. Finally,the strap spring is engaged with the heat sink. As explained above, theengagement can be semi-permanent or permanent, and in the latter case,permanent affixation can be obtained by mechanical engagement, such asbarbs.

Although the invention has been described in terms of an exemplaryembodiment, the spirit and scope of the appended claims are not to belimited by any details not expressly stated in the claims. Upon reviewof the foregoing, numerous alternative embodiments will presentthemselves to those of skill in the art. It will be realized that theprinciples set forth above are applicable to a wide variety ofsituations, and are not limited to the electronic devices illustrated,the types of heat sinks illustrated. Accordingly, reference should bemade to the appended claims in order to determine the full scope of thepresent invention.

What is claimed:
 1. A combination comprising: an electronic assemblyincluding a heat sink, a chip assembly, and a circuit board, said heatsink having a first and a second heat sink aperture formed therethrough,said circuit board having a circuit board contact side and a first and asecond circuit board aperture formed therethrough, said heat sinkcomprising a substantially planar heat sink contact side and having asurface larger than at least a portion of said chip assembly, said firstheat sink aperture in registration with said first circuit boardaperture, said second heat sink aperture in registration with saidsecond circuit board aperture, said chip assembly including a firstcontact side that contacts said heat sink contact side and a secondcontact side that is opposite said first contact side and that contactssaid circuit board contact side; and a strap spring including a bodyportion and a first and a second spring arm, said first and said secondspring arm extending from said body portion, said first spring armdefining a first end and said second spring arm defining a second end,said first spring arm end extending through said first circuit boardaperture and through said first heat sink aperture, said second springarm end extending through said second circuit board aperture and throughsaid second heat sink aperture, said body portion urging against a sideof said electronic assembly, each one of said first end and said secondend urging against an opposing side of said electronic assembly tocouple together said heat sink, said chip assembly, and said circuitboard, whereby said circuit board is not exposed to bending moments bysaid spring arm ends.
 2. The combination of claim 1 wherein said firstheat sink aperture is co-linear with said first circuit board aperture,and said second heat sink aperture is co-linear with said second circuitboard aperture.
 3. The combination of claim 1 wherein said heat sink,said chip assembly, and said circuit board are in layered contact. 4.The combination of claim 1 wherein said chip assembly includes amicroprocessor and a socket.
 5. The combination of claim 1 wherein eachone of said spring arms are flat between the body portion and the ends.6. The combination of claim 1 wherein said first end includes a firstengaging section and said second end includes a second engaging section,said strap spring holding each one of said heat sink, chip assembly, andcircuit board together in compression between said body portion and saidengaging sections.
 7. The combination of claim 6 wherein said firstengaging section includes a first mechanical coupler disposed proximatea distal tip of the first end and said second engaging section includesa second mechanical coupler disposed proximate a distal tip of saidsecond end, said strap spring holding each one of said heat sink, saidchip assembly, and said circuit board in compression between said bodyportion and said mechanical couplers.
 8. The combination of claim 6wherein said first and said second engaging sections each include amember for locking against said heat sink.
 9. The combination of claim 8wherein each one of said engaging sections comprises a flat undercutportion, whereby said strap spring can be disengaged from saidelectronic assembly.
 10. The combination of claim 8 wherein saidengaging sections are bent into an arcuate cross-section.
 11. Thecombination of claim 8 wherein said member comprises a barb forpermanently engaging said heat sink.
 12. The combination of claim 11wherein said barb comprises a section of said ends bent at an includedangle of less than 90° to create one or more barb points to engage saidheat sink.
 13. The combination of claim 6 wherein said two spring armsextend substantially flat and rectilinear between said contact sectionand said engaging sections.
 14. The combination of claim 6 wherein saidengaging sections extend from said body portion to form an acute angle.15. The combination of claim 6 wherein said chip assembly is disposedbetween said heat sink and said circuit board.
 16. The combination ofclaim 1, wherein the chip assembly is mounted directly on said circuitboard.
 17. The combination of claim 1 wherein said body portion of saidstrap spring urges against said circuit board opposite said circuitboard contact surface.
 18. The combination of claim 1 wherein the heatsink includes fins that extend from said heat sink opposite said circuitboard contact surface.